The present invention generally relates to increasing data throughput and quality in a wireless communication system and, more particularly, to systems and methods involving adaptation of soft handoff usage based on access network capacity in radiocommunication systems.
The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. At the same time usage of mobile communication equipment for transmission of data rather than speech has become increasingly popular by consumers. The ability to send and receive electronic mail and to use a web browser to obtain world-wide-web access is frequently discussed among services that will be used more and more in wireless communication systems. In response to this, communication system designers search for ways to efficiently transfer data information to and from mobile users and, in particular, to provide high data rate transfer capability.
Many radiocommunication systems have been designed in accordance with various standards, e.g., adopted on a country-wide or region-wide basis, in order to provide a roadmap for technological and service compatibility. For example, D-AMPS (IS-136) has been specified for North America, GSM for Europe and PDC for Japan. In considering ways to provide high data rate transfer capabilities, designers should also take into account existing standards in an effort to minimize the impact of design changes on the relevant standard(s) and legacy equipment.
However, costs associated with extending the capacity of the access network are severe. Accordingly, Applicants anticipate that, in some implementations, designers and/or network operators may opt to increase the throughput of the air interface without immediately providing sufficient access network capacity to handle a fully loaded air interface. Accordingly, it is desirable to find techniques for balancing capacity limits of the access network with other objectives of a radiocommunication system, e.g., reducing interference on the air interface. Such balancing techniques are also very interesting for CDMA systems.
In a typical CDMA system, an information data stream to be transmitted is impressed upon a much higher bit rate data stream produced by a pseudorandom code generator. The information signal and the pseudorandom signal are typically combined by multiplication in a process sometimes called coding or spreading the information signal. Each information signal is allocated a unique spreading code. A plurality of coded information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the coded signals overlap all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal can be isolated and decoded.
The need for transmit power control in the uplink is recognized in current CDMA cellular systems, as may be seen from xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d TIA/EIA Interim Standard TIA/EIA/IS-95 (July-1993) and its revision TIA/EIA Interim Standard TIA/EIA/IS-95-A (May 1995). Such standards that determine the features of U.S. cellular communication systems are promulgated by the Telecommunications Industry Association and the Electric Industries Association located in Arlington, Va.
Future Wideband CDMA systems will use fast downlink power control, i.e., power control based on quality measurements in the base station, and send power control commands to mobile stations to decrease or increase the output power towards a defined target. One such method is described in European Patent Publication No. 0 680 160 by Dohi et al.
While cellular systems were originally designed to operate with one-to-one correspondence between a mobile station and an associated base station covering a geographic cell, it has been determined that the effects of shadowing and fading can be reduced by communicating the same signal to a mobile station over more than one link. In systems using fast power control, like CDMA and wideband CDMA systems, a one-to-one correspondence would result in a low capacity compared to communication over more than one link. The capacity reduction will be particularly severe in the uplink. To overcome this, a multi-link communication can be established, both in the up- as well as the downlink, so that two or more base stations communicate the same information to a mobile station over two different spatially offset links. For the downlink this means that transmission occurs from two or more base stations and in the uplink two or more base stations listen to signals transmitted from mobile station. The mobile station processes the signals from the two links by selecting or combining them in some way, e.g., maximal ratio combining. This technique is known as diversity. Conventional spatial diversity techniques employ two or more. separated antennas in a single base station, or two or more base stations, to communicate with a mobile station. However, diversity is not limited to spatially offsetting base stations or antennas (i.e., multiple transmission paths). Diversity transmission can also be generated using one or more of an offset in time, polarization, or frequency.
One area in which macro diversity is commonly practiced is during handoff. In such cases, the candidate base station (i.e., the base station to which a mobile station is to be handed off) starts transmitting substantially the same message information to the mobile station before the current, serving base station terminates its transmission of that message information. This usage of macro diversity is commonly referred to as soft handoff. Soft handoff is described in U.S. Pat. No. 5,109,528 to Uddenfeldt and U.S. Pat. No. 5,327,577 to Uddenfeldt, both of which are expressly incorporated here by reference.
FIG. 1 illustrates a soft handoff arrangement wherein a first, original base station 202 and a second, candidate base station 204 each transmit a same message 206 to a mobile station 208. The message 206 is transmitted to the mobile station 208 over different signal paths in the forms of a first downlink 210 and a second downlink 212. The first and second downlink signals 210 and 212 are recombined (or one of the received signals is selected) in the mobile station 208 to extract the message 206. The mobile station 208 transmits to the base stations 202 and 204 over first and second uplink paths 214 and 216, respectively. At some point in time, the transmission of message information to the mobile station from the first, original base station 202 is terminated and the soft handoff process is concluded.
As transmission between the base station and Radio Access Network Nodes, e.g., RNC (Radio Network Controller), is expensive, it becomes evident that one drawback with soft handover is the increased load between the base station and the RNC. The total load between the RNC and the base stations associated with a single user is proportional to the number of base stations in its active set.
In a soft handoff arrangement, the base stations, and/or antennas communicating with a particular mobile station are known as xe2x80x9cactive setxe2x80x9d members. For example, referring back to FIG. 1, base stations 202 and 204 would be considered members of the active set. Those skilled in the art will appreciate that more than two base stations and/or antennas can be part of the active set. Members of an active set can change as the mobile station passes into and out of coverage areas handled by base stations and/or antennas in the system.
Soft handoff has been used in many different types of radiocommunication systems, including those using time division multiple access (TDMA) and code division multiple access (CDMA). Soft handoff increases robustness, achieves improved downlink quality, and combats fading. However, soft handoff may sometimes negatively impact system capacity and network resources due to the additional transmitting source(s) used to transmit substantially the same information to a receiver. For example, as described in the article entitled xe2x80x9cA Channel Assignment Scheme for Reducing Call Blocking rate in a DS-CDMA Cellular Systemxe2x80x9d by Hyoung-Goo Jeon et al., if all downlink codes are used, and a call needs to be placed, then a reverse traffic channel can be freed on the air interface by dropping a link being used in soft handoff. However, this article fails to recognize, as Applicants have, that capacity limitations may occur not only in the air interface, but in the ground-based access interfaces as well.
Accordingly, Applicants have determined that it would be desirable to balance the benefits of soft handoff in the air interface with the drawbacks of soft handoff in terms of access network capacity.
These and other drawbacks and limitations of conventional methods and systems for communicating information are overcome according to the present invention, wherein Applicants present techniques and systems to adjust soft handoff algorithms based on the access network""s current load. For example, according to one exemplary embodiment, when a transmission link between a base station and a radio network controller is substantially fully utilized, the soft handoff margin for new connections to this base station can be reduced.
According to another exemplary embodiment, when a transmission link between a base station and a radio network controller is substantially fully utilized, the soft handoff margin for both ongoing and new connections to this base station can be reduced. In this way, the active set of existing connections in soft handoff mode can be reduced so that capacity is freed in the access network to establish new connections.
According to another exemplary embodiment of the present invention, when a transmission link between a base station and a radio network controller is substantially fully utilized, no more than two members are allowed in the active set for new connections which are to enter soft handoff mode using this base station as a transmission source.
According to still another exemplary embodiment of the present invention, when a transmission link between a base station and a radio network controller is substantially fully utilized, no more than two members are allowed in the active set for both new and ongoing connections in soft handoff mode using this base station as a transmission source.